This invention relates to a humidity sensor for detecting and determining moisture in the surrounding atmosphere and a method for preparing the same.
Conventional humidity sensors are designed to detect humidity through changes of electrical properties, typically electric resistance. Known sensors use electrolytes such as lithium chloride, metal oxides, and organic polymers as the humidity sensitive material. However, the humidity sensors using electrolytes such as lithium chloride can measure only a narrow range of humidity and are less resistant to water in that their performance can be altered by dew condensation and wetting. The humidity sensors using metal oxides are resistant to water, but low sensitive. Because of the lack of long-term stability when used alone, they require a heat cleaning circuit which would add to the operating cost. The sensor structure is complex.
Among the humidity sensitive materials, organic polymers, especially polymeric electrolytes having quaternary ammonium salt groups have been widely used in commercial and industrial applications and so appreciated. For example, Japanese Patent Publication (JP-B) No. 61-54176 discloses a humidity sensitive material comprising aggregates of latex particles formed of a copolymer between a hydrophobic monomer and an ionic or non-ionic hydrophilic monomer and having a hydrophilic surface layer. There are exemplified some cationic compounds having primary to quaternary ammonium salts.
JP-B 62-7976 discloses a humidity sensitive material in the form of a polymer which is obtained by polymerizing a compound containing 2-hydroxy-3-methacryloxypropyl-trimethylammonium chloride to a degree of polymerization of 1,000 to 10,000.
JP-B 2-24465 discloses a humidity sensitive thin film of a polymer having the structural formula:
xe2x80x94(N+(R1)(R2)Xxe2x88x92-A-N+(R3)(R4)Xxe2x88x92-B)nxe2x80x94
wherein R1 to R4 are alkyl, Xxe2x88x92 is a halide ion, A and B each are xe2x80x94(CH2)mxe2x80x94 wherein mxe2x89xa72. The polymer may be blended with another polymer such as polyvinyl pyrrolidone for the purposes of improving substrate adhesion and water resistance. The blend is also effective in forming a humidity sensitive thin film.
Humidity sensors using the polymeric electrolytes exemplified above as the humidity sensitive material, however, are still low in water resistance in that the polymeric electrolytes can be partially leached in a high humidity region, especially in a dew condensing atmosphere. They also suffer from a hysteresis phenomenon that they produce different outputs at the same humidity depending on whether the humidity is increasing or decreasing. In a low humidity region having a relative humidity (RH) of less than 10%, they have so high resistance values that practical humidity measurement is impossible.
U.S. Pat. No. 5,546,802 (corresponding Japanese Patent No. 2,808,255 by TDK Corp.) discloses a humidity sensor comprising a polymer electrolyte having ethylenically unsaturated reactive groups introduced at ends and quaternary ammonium salt groups. This sensor is fully resistant to water, steadily operates in a condensing atmosphere and produces consistent outputs over a wide humidity region.
However, in order that the humidity sensor using a polymer electrolyte as defined above produce consistent outputs in a condensing atmosphere, the thickness of the humidity sensitive film must be restricted below a certain limit. Such a thin film is so sensitive to various gases that the output characteristics largely vary therewith. Therefore, the sensor has the drawback of lacking long-term stability under hot humid conditions. If the thickness of the humidity sensitive film is increased beyond the limit, stripping and failure are likely to occur.
A primary object of the present invention is to provide a humidity sensor device having a humidity sensitive thin film which is resistant to water, maintains effective, stable performance over a long time even in a dew condensing atmosphere, and produces accurate outputs in a stable manner over a wide humidity region.
In one aspect, the invention provides a humidity sensor comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate and defining a gap therebetween, a silicon-containing undercoat layer lying on at least the gap, and a humidity sensitive thin film lying thereon. The humidity sensitive thin film comprises a crosslinked product of a conductive polymer having ethylenically unsaturated groups and is physically bound to the undercoat layer through an interpenetrating polymer network.
In another aspect, the invention provides a humidity sensor comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate and defining a gap therebetween, a silicon-containing undercoat layer lying on at least the gap, and a humidity sensitive thin film lying thereon. The humidity sensitive thin film comprises a crosslinked product of a polymer of the following formula (I) and is bound to said undercoat layer through covalent bonds having not undergone dehalogenation reaction. 
Herein A and B each are a divalent group; Y1, Y2, Y3, Y4, Y5 and Y6 each are a monovalent group, at least one of Y1 to Y6 is a group terminated with an ethylenically unsaturated reactive group; at least two of Y1, Y2, Y3, Y4, Y5, A and portions thereof adjoining the nitrogen (N) atom or at least two of Y4, Y5, Y6, B and portions thereof adjoining the nitrogen (N) atom, taken together, may form a ring with the nitrogen atom; Xxe2x88x92 is a halide ion; and letter n is a number of 2 to 5,000.
Preferably, the humidity sensitive thin film of the second embodiment is physically bound to said undercoat layer through an interpenetrating polymer network.
Typically, the humidity sensitive thin film has a thickness of 0.1 to 20 xcexcm.
In a preferred embodiment, the polymer has the following formula (II) or (III). 
Herein A and B each are a divalent group; each of R1, R2, R3, and R4 is an alkyl or alkenyl group; a pair of R1 and R2, R1 and A or a portion of A, R2 and A or a portion of A, R3 and R4, R3 and A or a portion of A, R4 and A or a portion of A, R1 and R3, R1 and R4, R2 and R3, or R2 and R4, taken together, may form a ring with the nitrogen (N) atom; L is a divalent group; R is hydrogen or an alkyl group; Xxe2x88x92 is a halide ion; n is a number of 2 to 5,000; and R5 and R6 each are an alkyl or alkenyl group.
Preferably, the divalent group represented by A is an alkylene, alkenylene or arylene group or a mixture thereof; the divalent group represented by B is an alkylene, alkenylene or arylene group in which at least one of an oxy group (xe2x80x94Oxe2x80x94) and a carbonyl group (xe2x80x94COxe2x80x94) may intervene or a mixture thereof.
The polymer is typically obtained by reacting a diamine compound with a dihalogen compound to form an intermediate polymer and introducing an ethylenically unsaturated reactive group into the intermediate polymer at each end.
In a further aspect, the invention provides a method for preparing a humidity sensor comprising the steps of:
applying a silane compound containing at least a hydrolyzable group and an organic group having an unsaturated bond onto a substrate to form an undercoat layer,
applying a solution containing a conductive polymer having ethylenically unsaturated groups onto the undercoat layer, and
exposing the conductive polymer to radiation for crosslinking the polymer and bonding the polymer with the silane compound to thereby form a humidity sensitive thin film.
Preferably, the bond between the conductive polymer and the silane compound is due to covalent bonds by crosslinking between the ethylenically unsaturated groups and the unsaturated bonds. Most often, the radiation is ultraviolet radiation. Preferably the conductive polymer used herein has the formula (I) defined above. 
Herein A and B each are a divalent group; Y1, Y2, Y3, Y4, Y5 and Y6 each are a monovalent group, at least one of Y1 to Y6 is a group terminated with an ethylenically unsaturated reactive group; at least two of Y1, Y2, Y3, Y4, Y5, A and portions thereof adjoining the nitrogen (N) atom or at least two of Y4, Y5, Y6, B and portions thereof adjoining the nitrogen (N) atom, taken together, may form a ring with the nitrogen atom; Xxe2x88x92 is a halide ion; and letter n is a number of 2 to 5,000.
In a preferred embodiment, the silane compound has the following formula (IV):
X0n(CH3)3xe2x88x92nSi-R0xe2x80x83xe2x80x83(IV) 
wherein X0 is a hydrolyzable group, R0 is an organic group, and n is an integer of 1, 2 or 3.
In the humidity sensor of the invention, an undercoat layer having unsaturated bonds connected via silicon covers a pair of opposed electrodes on an insulating substrate, and a humidity sensitive layer of a conductive polymer of any of formulas (I) to (III) lies thereon.
The undercoat layer comprising a compound having an unsaturated bond connected via silicon, preferably of formula (IV) is provided for the purpose of establishing a firm or intimate bond between the overlying humidity sensitive layer of conductive polymer and the underlying substrate and electrodes. The compound is typically selected from silane coupling agents or silane compounds having vinyl, methacrylic or acrylic groups.
The humidity sensitive layer contains a crosslinked product of a polymer represented by formula (I), preferably formula (II) or (III). This layer is formed by applying a solution of the polymer and causing the polymer to be crosslinked, preferably by exposure to UV radiation. During the crosslinking step, the polymer forms covalent bonds or physical bonds with the undercoat layer having unsaturated bonds connected via silicon. As a consequence, there is formed a robust humidity sensitive film.
Upon exposure to UV radiation, the ethylenically unsaturated groups on the conductive polymer and the unsaturated double bonds on the compound are polymerized and mutually crosslinked within the respective layers and at the interface therebetween. At this point, in the conductive polymer layer, the humidity sensitive material becomes insolubilized in water by polymerization. At the interface with the undercoat layer having unsaturated bonds connected via silicon, covalent bonds are formed via silicon to bind the humidity sensitive layer of conductive polymer to the electrodes and insulating substrate through the undercoat layer. Alternatively, the humidity sensitive layer is closely bonded to the electrode and insulating substrate through the undercoat layer due to covalent bonds by crosslinking between ethylenically unsaturated groups and unsaturated bonds and physical bonds by an interpenetrating polymer network (IPN) resulting from crosslinking between unsaturated bonds. By virtue of this phenomenon, the humidity sensitive layer is firmly bound to the electrode and insulating substrate through the undercoat layer so that the humidity sensitive film has improved water resistance and wear resistance. The humidity sensor is fully resistant to water. The humidity sensing function is not lost at all, albeit crosslinking.
The interpenetrating polymer network (IPN) means that distinct polymer molecules of a three-dimensional network component are physically intertwined without forming chemical bond sites so that they penetrate into different phases in a network manner. As seen from formulas (I) to (III), the polymer of the conductive polymer layer is structurally characterized by containing a quaternary ammonium salt group (including a cyclized one) in its backbone and having an ethylenically unsaturated reactive group at one or both, preferably both of the terminal ends of the polymer.
In the humidity sensitive thin film containing the specific polymer, the quaternary ammonium salt group moiety of the polymer molecule contributes to electric conductivity and the counter ion thereto is dissociated with moisture in the surrounding atmosphere to develop ionic conduction. Humidity is detected by utilizing the phenomenon that the degree of dissociation varies as the moisture content in the atmosphere increases or decreases.
Since the polymer contains a quaternary ammonium salt group in its backbone, the humidity sensor produces accurate outputs without hysteresis.
It is noted that JP-A 7-318525 discloses a humidity sensor comprising a polymer electrolyte bound to a substrate surface through covalent bonds containing xe2x80x94Sixe2x80x94 bonds. As opposed to the present invention, the layer between the substrate and the humidity sensitive film is a silane compound having halide groups, and the covalent bonds between the compound layer and the humidity sensitive material results from dehalogenation reaction. It is described nowhere that the crosslinking of a humidity sensitive material and the crosslinking between a humidity sensitive material and a silane compound are induced by exposure to radiation. In these regards, the humidity sensor of JP-A 7-318525 is apparently different from the humidity sensor of the present invention.
JP-B 7-018832 discloses a humidity sensor comprising a humidity sensitive film based on an alkoxysilane having quaternary ammonium groups. An organopolysiloxane has quaternary ammonium groups, which are chemically bonded by heating. It is described nowhere that the crosslinking of a humidity sensitive material and the crosslinking between a humidity sensitive material and a silane compound are induced by exposure to radiation. In these regards, the humidity sensor of JP-B 7-018832 is apparently different from the humidity sensor of the present invention.